The invention pertains to protecting and reducing the recovery time of an AC coupled magnetoresistive read head amplifier for a disc drive. More particularly, it pertains to a preamplifier bias circuit.
In magnetic disc storage systems, it is common to employ MR elements to transduce data recorded on the disc surface to electrical signals for amplification and processing by the data processing apparatus. The MR element is mounted to an actuator arm to be selectively positioned adjacent various concentric tracks on the disc surface. Usually, a separate MR element is employed for each disc recording surface so that where both sides of a disc are recording surfaces, two MR elements are employed. It is also common to stack plural discs for common rotation about a single spindle axis. Thus, a stack of six discs might employ as many as twelve recording surfaces and twelve MR elements.
Ordinarily, data are read from a single disc surface at any given time. Consequently, it is desirable that the bias current to the MR elements be selectively applied only to that element currently active. Data on the recording surface are transduced by the MR element which changes its impedance in accordance with changes in magnetic flux from data recorded on the surface. This change in impedance alters the voltage across the MR element caused by the bias current, and the changing voltage then is applied to an amplifier for processing.
It will be appreciated that the signal applied to the amplifier is an alternating current (AC) signal. However, due to the constant MR head bias current and finite resistance of the MR element, the signal also contains a direct current (DC) offset which adversely affects the recovery of data. For this reason, it is common to employ coupling capacitors to block the DC signal from the MR element so that only the AC signal is applied to the amplifier. Ordinarily, these coupling capacitors are quite large with a separate set of coupling capacitors employed for each of the read channels. Additionally, a separate read amplifier or a portion of the read amplifier is also employed for each read channel.
In order for the coupling capacitors to be effective in blocking the DC signal from the MR element, the charge on the coupling capacitors must be equilibrated with the MR element's DC potential. This equilibration process generally occurs when the bias current is first applied to an MR element but can also occur when transient signals propagate through the system such as when a thermal asperity occurs or when a write-to-read transition occurs. The length of time required to equilibrate the coupling capacitors represents a time delay to the magnetoresistive read head amplifier becoming operational and is equal to the settling time constant of the coupling capacitors. The equilibration time period is therefore equal to the product of the coupling capacitor and the impedance of the read amplifier and its bias circuitry. Thus, in order to decrease the time delay associated with the magnetoresistive read head amplifier becoming operational, the setting time constant of the coupling capacitors must be minimized.
A further concern in utilizing MR elements with read amplifiers in disc drive designs is that transient signals can flow across and damage MR elements when they are not selected for operation. Preferably, the unselected MR elements are held at a known potential which is equal to the potential of the disk. Otherwise, if a large potential difference is created between the disk and the MR element, then a spark can be generated therebetween causing damage to both the disk media and the head element. Thus, a need exists to protect and decrease the time delay of an AC coupled magnetoresistive read head amplifier for a disc drive.